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Trimethoprim - Brand names: Monotrim, Minotrim, Trimogal.

On this page

  1. About trimethoprim
  2. Key facts
  3. Who can and cannot take trimethoprim
  4. How and when to take trimethoprim
  5. Side effects
  6. How to cope with side effects of trimethoprim
  7. Pregnancy and breastfeeding
  8. Cautions with other medicines
  9. Common questions about trimethoprim

1. About trimethoprim

Trimethoprim is an antibiotic.

It's used to treat and prevent urinary tract infections (UTIs), such as cystitis.

Occasionally, trimethoprim is used to treat other types of infections, such as chest infections and acne.

Trimethoprim is available on prescription. It comes as tablets and as a liquid that you swallow.

2. Key facts

  • You'll usually take trimethoprim twice a day to treat infections.
  • For most infections, you'll feel better within a few days. But it's important to finish your course of trimethoprim to help stop your infection from coming back.
  • Side effects may include itching or a skin rash, but these are usually mild and short-lived.
  • You can drink alcohol while taking trimethoprim.
  • Tell your doctor if you do not start feeling better after taking trimethoprim for 3 days, or if you start to feel worse at any time.

3. Who can and cannot take trimethoprim

Most adults and children can take trimethoprim.

Trimethoprim is not suitable for some people. To make sure this medicine is safe for you, tell your doctor if you:

  • have ever had an allergic reaction to trimethoprim or any other medicine
  • have liver or kidney problems
  • have anaemia or low amounts of folic acid (folate) in your blood
  • have porphyria (a rare inherited blood disorder) or any other blood disorder
  • are trying to get pregnant or are already pregnant

4. How and when to take trimethoprim

Always follow your doctor or pharmacist's advice, and the instructions that come with your medicine.

Dosage and strength

Trimethoprim tablets contain 100mg or 200mg. The liquid contains 50mg in 5ml.

The usual dose of trimethoprim to:

  • treat urinary tract infections (UTIs) is 200mg twice a day – your doctor might recommend you double the first dose to 400mg
  • prevent infections is 100mg once a day
  • treat cystitis that comes on after having sex is a single dose of 100mg taken within 2 hours of having sex
  • treat acne is 300mg twice a day – this dose might be reduced over time

The dose of trimethoprim you need to take depends on your condition, your age, and how well your kidneys work.

Doses are usually lower for children, older people and those with kidney problems.

Important: Completing your course of trimethoprim

Carry on taking this medicine until you finish the course, even if you feel better. If you stop your treatment early, your problem could come back.

How to take it

You'll usually take trimethoprim twice a day to treat an infection, once in the morning and once in the evening.

You can take it with or without food.

Swallow trimethoprim tablets whole with a drink of water. Do not chew or break them.

Trimethoprim is available as a liquid for people who find it difficult to swallow tablets.

If you're taking trimethoprim as a liquid, shake the bottle gently before measuring out the right amount using a medicine spoon or oral syringe. If you do not have an oral syringe or medicine spoon, ask your pharmacist for one. Do not use a kitchen teaspoon as it will not measure the right amount.

If you're taking trimethoprim to prevent an infection, take it at bedtime.

If you have been prescribed trimethoprim as a treatment for cystitis that comes on after having sex, take it as a single dose within 2 hours of having sex.

How long to take it for

The length of time you'll need to take trimethoprim for depends on how bad and where your infection is, your age, whether you're male or female, and whether you have any other health problems.

Women and children with straightforward UTIs usually take a 3-day course of treatment.

Men and pregnant women with straightforward UTIs usually take a 7-day course of treatment.

People with particularly severe or complicated UTIs, or a catheter, usually take a 14-day course of treatment.

Men with a UTI that causes swelling of the prostate gland (prostatitis) may need a treatment course for 4 to 6 weeks.

People taking it to prevent UTIs may need to take it for at least 6 months.

People taking it for acne may need to take it for at least 6 months.

It's very important to keep taking trimethoprim until your course is finished, even if you feel better. This helps stop the infection coming back.

If you forget to take it

If you forget to take a dose of trimethoprim, take it as soon as you remember, unless it's nearly time for your next dose. In this case, just leave out the missed dose and take your next dose at the usual time.

Never take 2 doses at the same time. Never take an extra dose to make up for a forgotten one.

If you often forget doses, it may help to set an alarm to remind you. You could also ask your pharmacist for advice on other ways to help you remember to take your medicine.

If you take too much

Taking an extra dose of trimethoprim is unlikely to harm you, but it may increase the chances of temporary side effects, such as feeling or being sick and diarrhoea.

Urgent advice: Contact 111 for advice now if:

You've taken more than your prescribed dose of trimethoprim and:

  • you're worried or get severe side effects
  • you've taken more than 1 extra dose

Go to or call 111

Call 111 if you're asking about a child under the age of 5 years.

5. Side effects

You're unlikely to get side effects from trimethoprim. Some people get itching or a skin rash, but this is usually mild and goes away after you stop taking the medicine.

Common side effects

These common side effects of trimethoprim happen in more than 1 in 100 people. Speak to your doctor or pharmacist if these side effects bother you or do not go away:

  • itching or a mild rash
  • feeling or being sick (nausea or vomiting)
  • diarrhoea
  • headaches

Serious side effects

Most serious side effects are rare and happen in less than 1 in 1,000 people.

Call a doctor or call 111 straight away if you have:

  • diarrhoea (possibly with stomach cramps) that contains blood or mucus, or severe diarrhoea that lasts longer than 4 days
  • bruising or bleeding you cannot explain (including nosebleeds), a sore throat, mouth ulcers, a high temperature, or you feel tired or generally unwell – these can be signs of a problem with your blood

Immediate action required: Call 999 or go to A&E now if:

  • you have headaches, a high temperature, a stiff neck, tiredness, feel ill, and your eyes become very sensitive to bright light – these can be signs of meningitis which is a very rare side effect of trimethoprim
  • you have muscle weakness, an abnormal heartbeat or chest pains – these can be signs of high potassium in your blood

Serious allergic reaction

In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to trimethoprim.

Immediate action required: Call 999 now if:
  • your lips, mouth, throat or tongue suddenly become swollen
  • you're breathing very fast or struggling to breathe (you may become very wheezy or feel like you're choking or gasping for air)
  • your throat feels tight or you're struggling to swallow
  • your skin, tongue or lips turn blue, grey or pale (if you have black or brown skin, this may be easier to see on the palms of your hands or soles of your feet)
  • you suddenly become very confused, drowsy or dizzy
  • someone faints and cannot be woken up
  • a child is limp, floppy or not responding like they normally do (their head may fall to the side, backwards or forwards, or they may find it difficult to lift their head or focus on your face)

You or the person who's unwell may also have a rash that's swollen, raised, itchy, blistered or peeling.

These can be signs of a serious allergic reaction and may need immediate treatment in hospital.

These are not all the side effects of trimethoprim. For a full list, see the leaflet inside your medicine packet.


You can report any suspected side effect using the Yellow Card safety scheme.

Visit Yellow Card for further information.

6. How to cope with side effects of trimethoprim

What to do about:

  • itching or a mild rash – it may help to use an emollient cream or gentle, fragrance-free moisturiser, or take an antihistamine, which you can buy from a pharmacy. Check with the pharmacist to see what may be suitable for you.
  • feeling or being sick – try taking trimethoprim with or after food to see if that helps ease your symptoms. It may also help if you stick to simple meals and avoid rich or spicy food while you're taking this medicine. If you're being sick, try small frequent sips of water to avoid dehydration.
  • diarrhoea – drink plenty of water or squash to avoid dehydration. Speak to a doctor if you have signs of dehydration such as peeing less than usual or having dark, strong-smelling pee. It may also help to take oral rehydration solutions which you can buy from a pharmacy. Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor.
  • headaches – make sure you rest and drink plenty of fluids. It's best not to drink too much alcohol. Everyday painkillers, such as paracetamol and ibuprofen, are safe to take with trimethoprim. Speak to your doctor if these do not help or the headaches are severe.

7. Pregnancy and breastfeeding

Trimethoprim and pregnancy

Trimethoprim can be taken in pregnancy, but it is not recommended in the first 12 weeks. This is because it may affect your levels of folic acid, which is important in the early stages of your baby's development.

Sometimes trimethoprim is the only suitable antibiotic to treat an infection. In this case the benefit of taking it is likely to outweigh the small risk of harm, but discuss this with your doctor.

If you do need to take trimethoprim while you're trying to get pregnant or during the first 12 weeks of pregnancy, your doctor may also recommend taking high dose folic acid (5mg per day).

It's OK to take trimethoprim in later pregnancy (after 12 weeks) and there's no particular need to take high dose folic acid with it, although it will not do any harm if you continue to take it.

Trimethoprim and breastfeeding

If your doctor or health visitor says your baby is healthy, you can take trimethoprim while breastfeeding.

Trimethoprim passes into breast milk in small amounts and is unlikely to cause side effects in your baby. But it's best to take trimethoprim only for a short time because it may affect your baby's folic acid levels. If you need to take trimethoprim for longer than a few weeks, talk to your doctor or pharmacist.

If your baby is not feeding as well as usual, has a stomach upset, or has oral thrush (a fungal infection in their mouth), or if you have any other concerns about your baby, talk to your doctor, pharmacist, health visitor or midwife.

Non-urgent advice: Tell your doctor if you're:

  • trying to get pregnant
  • pregnant
  • breastfeeding

For more information about how trimethoprim can affect you and your baby during pregnancy, visit the Best Use of Medicines (BUMPS) website.

8. Cautions with other medicines

There are many medicines that do not mix well with trimethoprim.

Tell your doctor or pharmacist if you're taking any other medicines before starting to take trimethoprim, in particular:

  • rifampicin, an antibiotic
  • warfarin or acenocoumarol (Sinthrome), medicines to treat or prevent blood clots (anticoagulants)
  • digoxin, a heart medicine
  • phenytoin, an epilepsy medicine
  • replaglinide or pioglitazone, diabetes medicines

Typhoid vaccines given by mouth may not work properly if you're taking trimethoprim. It does not affect typhoid vaccines given by injection.

Mixing trimethoprim with herbal remedies and supplements

It's not possible to say that complementary medicines or herbal remedies are safe to take with trimethoprim. They're not tested in the same way as pharmacy and prescription medicines.

Important: Medicine safety

Tell your doctor or pharmacist if you're taking any other medicines, including herbal medicines, vitamins or supplements.

9. Common questions about trimethoprim

How does trimethoprim work?

Trimethoprim kills bacteria by stopping them making a substance called folic acid, which they need to survive.

This is why trimethoprim can sometimes cause low folic acid levels in your blood over time.

When will I feel better?

You should feel better within a few days.

It's important that you keep taking trimethoprim until your course is finished.

Do this even if you feel better. It will help stop the infection coming back.

What if I do not get better?

Tell your doctor if you do not start feeling better after taking trimethoprim for 3 days, or if you start to feel worse at any time.

Will it give me thrush?

Some people get a fungal infection called thrush after taking a course of antibiotics like trimethoprim.

It happens because antibiotics kill the normal harmless bacteria that help to protect you against thrush.

Ask your pharmacist or doctor for advice if this happens to you.

Can I drive or ride a bike?

Yes. Trimethoprim should not affect you being able to drive or cycle.

Will it reduce my fertility?

There's no evidence to suggest that taking trimethoprim reduces fertility in either men or women.

But if you're trying to get pregnant and you are taking trimethoprim every day, talk to your doctor, as this medicine is not usually recommended in early pregnancy. They may recommend changing to another antibiotic, or they may also prescribe high dose folic acid (5mg per day).

Will it stop my contraception working?

Trimethoprim does not stop any type of contraception from working, including the combined pill and emergency contraception.

But if trimethoprim makes you sick or have severe diarrhoea for more than 24 hours, your contraceptive pills may not protect you from pregnancy. Look on the pill packet to find out what to do.

Read more about what to do if you're on the pill and you're being sick or have diarrhoea.

Can I drink alcohol with it?

Yes, you can drink alcohol with trimethoprim. But it's best to keep to the national guidelines of no more than 14 units a week.

Is there any food or drink I need to avoid?

You can eat and drink normally while taking trimethoprim.

Does cranberry juice help urinary tract infections?

It's unlikely to do any harm, but there's little evidence that drinking cranberry juice or taking cranberry supplements helps treat or prevent urinary tract infections.

Check with a pharmacist to make sure it's safe for you.

Can lifestyle changes help urinary tract infections?

It's possible that a straightforward urinary tract infection will clear up on its own without any treatment. But it's usually best to treat a urinary tract infection with an antibiotic or it could spread to the kidneys and lead to more serious problems.

Once treated, there are many steps you can take to stop urinary tract infections coming back:

  • avoid perfumed bubble bath, soap or talcum powder around your genitals – use plain, unperfumed varieties, and have a shower rather than a bath
  • go to the toilet as soon as you need to pee, and always empty your bladder fully
  • stay well hydrated – aim to drink 6 to 8 glasses of fluid a day (water, lower fat milk and sugar-free drinks, including tea and coffee, all count)
  • wipe your bottom from front to back when you go to the toilet
  • empty your bladder as soon as possible after having sex
  • do not use a contraceptive diaphragm or condoms with spermicidal lubricant on them – use another type of contraception instead
  • wear underwear made from cotton, rather than synthetic material like nylon
  • avoid tight jeans and trousers